1. Field of the Invention
This invention relates to the structure of gyroscopes and in particular to the structure of the rotor and windings of a two-axis gyroscope such as the gyroscope of a seeker missile. Specifically the invention concerns effecting very smooth precession of the gyroscope by means of a rotor configuration which permits DC excitation of the precession windings.
2. Description of the Prior Art
A variety of gyroscopically stabilized platforms have been used in target seeker missiles. A gyroscope used in one such target seeking missile is disclosed in U.S. Pat. No. 4,093,154 issued to McLean on June 6, 1978. A copy of that patent is enclosed. As is typical with gyroscopes of the prior art (so far as is known) that gyroscope requires the use of an AC signal to accomplish precession of the gyroscope. The AC signal is required because precession is accomplished by interaction of the current in the precession coil with a magnetic field (produced by magnet 22 of McLean) which changes with rotation of the rotor assembly. Because the direction of precession is determined by the rotational position of the rotor magnet, precession in a given desired direction can only be achieved when the rotor magnet is in a corresponding determinable position. Hence the precession control current is applied only when the rotor magnet reaches the determinable position. Precession is therefore not smooth, but performed in a series of pulsed precessions.
In any gyroscope where precession and spin are accomplished by interaction of the precession control current and the spin control current with the same magnetic field or fields which change as the rotor rotates, cross coupling between spin and precession occurs. This same configuration will also produce a response in the precession axes that is not being commanded. This is commonly known as wobbulation. An AC precession control signal and an AC servo will thus be required to effect precession of the rotating rotor magnet. In the gyroscope disclosed in McLean, gyro spin (motoring) is accomplished by directing compressed gas against vanes on the rotor instead of by an electrical coil and permanent magnet. Thus, the gyroscope disclosed in the McLean patent does not exhibit perturbations in precession due to any electrical current which in other gyroscopes (but not in the McLean gyroscope) controls gyro spin. Nonetheless, since the McLean gyro uses a rotating magnetic field to effect precession, the resulting precession is not as smooth as that effected by the gyro configuration of the instant invention.
In those gyroscopes which accomplish motoring and precession by electrical fields and permanent magnets, cross coupling is a relevant consideration. A certain amount of cross coupling results whenever the motor magnets are also used to effect precession. It is thus a primary objective of the invention to provide a gyroscope platform having minimal cross coupling between the motoring control signal and the resulting gyroscope precession. Minimal cross coupling will result in a very smoothly precessible gyroscope since no precession would result from the AC motoring signal.
In most gyroscopes employing an AC servo to effect precession, the amount of cross coupling (i.e. the effect of the motoring signal on the precession of the gyroscope) is dependent upon the gimbal angles. The greater the angular displacement of the gyroscope from the missile body axes, the greater the cross coupling. If cross coupling could be made independent of the gimbal angles, the gyroscope could withstand greater gimbal angles and still provide acceptable performance and accuracy.
Some examples of known seeker missiles include the Sidewinder Missile seeker, Redeye, Agile (Hughes' Mod 1 and Mod 2) and the Laser Maverick seekers (Rockwell). In each of these, the rotor has a single magnetic axis. Because the magnetic vector changes with respect to the non-rotating torquer (precession) coil with the change in rotor position, an AC servo is required. Since both the motoring coils (which produce gyro spin) and torquing coils (which produce precession) interact with the same rotor magnetic field vector, unwanted magnetic cross coupling is produced. Since the cross coupling increases with increased gimbal angles, this arrangement limits the gyro to relatively small gimbal angles.